1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a copier, a fax machine or the like, and more particularly to an image forming apparatus in which a cleaning member cleans transfer residual toner adhering to the surface of a charging member that charges a carrier.
2. Description of the Related Art
Known conventional image forming apparatuses include apparatuses in which a photosensitive element is charged through contact of the surface of the photosensitive element with a charging roller to which voltage is applied. Since the charging roller comes thus into contact with the photosensitive element, toner remaining on the surface of the photosensitive element, after cleaning of the photosensitive element surface by a cleaning brush or the like, becomes adhered to the surface of the charging roller. When toner becomes thus adhered to the surface of the charging roller, the photosensitive element cannot be appropriately charged at the portions where toner is adhered, which gives rise to non-uniform charging of the photosensitive element surface. The toner adhered to the surface of the charging roller must therefore be removed.
In the image forming apparatus described in, for instance, Japanese Unexamined Patent Application Laid-open No. 2003-316130, comprising two or more cleaning brushes for cleaning a charging roller, at least one of the cleaning brushes does not come into contact with the charging roller while the remaining cleaning brushes do so, to remove thereby toner adhered to the surface of the charging roller.
When the cleaning brush removes toner adhered to the surface of the charging roller, toner accumulates gradually on the cleaning brush, eventually impairing the cleaning performance of the cleaning brush. When its cleaning performance becomes impaired, the cleaning brush fails to clean the charging roller well, which gives rise, as described above, to non-uniform charging of the photosensitive element surface.
In the image forming apparatus described in Japanese Unexamined Patent Application Laid-open No. 2003-316130, as a result, the charging roller can be charged well by switching a cleaning brush having lessened cleaning performance to a cleaning brush having no lessened cleaning performance, for instance when the surface potential of the photosensitive element is reduced. The toner accumulated in the cleaning brush with lessened cleaning performance is knocked off by a knocking member, to restore thereby the cleaning performance of the cleaning brush with lessened cleaning performance.
However, the following problems arise in the image forming apparatus described in Japanese Unexamined Patent Application Laid-open No. 2003-316130. Specifically, cleaning of the charging roller by the cleaning brush is deficient immediately prior to cleaning brush switching, and hence there occurs non-uniform charging of the surface of the photosensitive element by the charging roller immediately prior to the above switching. As a result, an image formed on the photosensitive element that is charged immediately prior to the above switching exhibits, for instance, uneven density during printing, when the surface potential of the photosensitive element decreases. Moreover, there must be provided a waste toner tank or the like for recovering the toner knocked off the cleaning brush, which entails a larger size of the apparatus main body that accommodates such a waste toner tank.